1. Field of the Invention
The present invention relates to radio receivers generally and more particularly to a GPS satellite receiver employing means for computing heading information.
2. Description of the Prior Art
The NAVASTAR Global Positioning System (GPS) is a U.S. Defense Department satellite-based radio-navigation system transmitting information from which extremely accurate navigational information can be computed including the time, the user's three-dimensional position anywhere on or near the Earth, and the user's three-dimensional velocity. When fully operational, the GPS is planned to employ 18 satellites evenly dispersed in three, inclined, 12-hour circular orbits chosen to insure continuous 24-hour coverage worldwide. Each satellite carries extremely accurate cesium and rubidium vapor atomic clocks providing timing information. Additionally, each satellite is provided clock correction and orbital information by Earth-based monitoring stations.
Each satellite transmits a pair of L-band carrier signals including an L1 signal having a frequency of 1575.42 MHz (also referred to as 1540 f0 where f0 is 1.023 MHz) and an L2 signal having a frequency of 1227.6 MHz (1200 f0). The L1 and L2 signals are biphase modulated by pseudo-random noise (PRN) codes. The PRN codes facilitate multiple access. Since each satellite uses different PRN codes, a signal transmitted by a particular satellite can be selected by generating and matching (correlating) the corresponding PRN code pattern. Additionally, the PRN codes facilitate signal transmit time measurements which can be made by measuring the phase shift required to match the code. Both of the carrier signals (L1 and L2) are modulated by a PRN code which is referred to as a precision (p) code. The p PRN code, which is intended for military purposes, is a relatively long, fine-grained, precision code having a clock rate of 10.23 MHz (10 f0). The L1 carrier signal is additionally modulated by a PRN code which is referred to as a clear/acquisition (C/A) code. The C/A PRN code, which is intended for rapid signal acquisition and for commercial purposes, is a relatively short, coarsegrained code having a clock rate of 1.023 MHz (f0) and a code length of 1023 bits (one ms). A full bit (chip) of C/A PRN code, phase delay corresponds to a distance of 293 meters. In addition to the PRN codes, both of the signals (L1 and L2) are, continuously, biphase modulated by a 50 bit per second, 1500 bit long, navigation data bit stream. The navigation data bit stream includes information as to the status and emphemeris of all satellites, parameters for computing the particular satellite clock, and corrections for atmospheric propagation delays.
Disclosed in the U.S. Patent Application Ser. No. 06/607,824 filed May 7, 1984 now U.S. Pat. No. 4,754,465 of Charles R. Trimble is a Global Positioning System Course Acquistion Code Receiver suitable for computing the position and velocity information. However, the above mentioned receiver, as disclosed, lacks means for computing heading information (including yaw and pitch or roll).